1. Field of the Invention
This invention relates to an apparatus for detecting a light signal drowned in noises.
2. Description of the Prior Art
A light signal detecting apparatus is known in which light as a signal is emitted from a light emitting portion, is received by a light receiving element disposed several meters to ten-odd meters apart from the light emitting portion, is converted into an electric signal, and is amplified for detection. In this conventional apparatus, modulation light of which the duty ratio is 50% is used as the light signal. Further, in the apparatus, a narrow-band filter, a phase-lock loop, etc., are used to enhance the detectability of the light signal.
However, the apparatus needs to receive a light signal larger in amplitude than noises peculiar to the apparatus or noises caused by disturbance light in spite of the enhancement of the detectability by means of, for example, the narrow-band filter and the phase-lock loop. In other words, the apparatus makes it a condition that the light signal received by the light receiving element is in a state of separation from noises, and therefore the light emitting portion needs to deliver a high output. That is, a fault in the conventional apparatus is that it is difficult to detect the light signal buried or drowned in the noises (i.e., light signal having a signal-to-noise ratio below 1).
It is therefore an object of the present invention to provide a light signal detecting apparatus capable of detecting a light signal having a signal-to-noise ratio (S-N ratio) below 1.